The Delmhorst Concrete Sleeve & Plug is 4" long and come in a pack of 10. The Sleeve is placed inside a pre-drilled hole in a concrete slab, with the cap on, and left there for 72 hours. After that time period remove the cap and place the sensor inside the sleeve which is inside the concrete slab to obtain a reading. The sleeves are not reusableand are thrown away after use. These sleeves protect the sensor from getting damaged while in the concrete.